LPG contributes to the prevention of acid rain because of a low sulfur content thereof, and the burned gas contains a relatively small amount of CO2 as compared with petroleum and coal. Therefore, LNG is considered as an ecological, clean energy source. LNG can be liquefied under a relatively low pressure, and the liquefied volume thereof is a 250th of the original gas volume. Furthermore, LNG can be stored and transported in a tank which is capable of withstanding a relatively low pressure. Currently, steel pressure vessels are commonly used for storing and transporting LNG in Japan, but light weight pressure vessels made of composite materials based on plastic technology are widely used in western countries. The composite pressure vessel typically comprises a plastic liner, a fiber reinforced plastic layer (FRP layer) reinforcing the outer surface of the plastic liner and a metallic mouth member that protrudes outward from the FRP layer and defines an opening for introducing and expelling LPG into and out of the pressure vessel.
The LPG pressure vessel made of steel for home use is required to withstand the pressure of 3 MPa (30.59 kgf/cm2), and to be free from any LNG leakage at this pressure. In a composite pressure vessel, the metallic mouth member is required to be bonded to the plastic liner or to the FRP layer, and various proposals have been made to prevent leakage from the bonding interface (See Patent Documents 1 and 2, for instance).
The prior inventions disclosed in these patent documents feature special physical configurations of the plastic liner and the mouth member. However, the bonding between plastic and metal parts involves some difficulty. It often is the case that leakage can be prevented when the internal pressure of the vessel is relatively high, but some leakage occurs when the interval pressure is low.